1. Field of the Invention
The present invention relates to a gas sensor capable of detecting trace gas
2. Description of the Related Art
There are developed gas sensors for purposes of detecting gas leakage of methane gas (CH4), propane gas (C3H8), carbon monoxide (CO) and the like, and imperfect combustion, as well as for detecting and analyzing volatile organic compounds (VOC).
When a gas sensor is used as a gas leak detector for methane gas or the like, the gas sensor has only to detect the gas in the order of 1000 to 10000 ppm below the explosion limit, and in the case of imperfect combustion, a gas sensor has only to detect the gas in the order of 10 to 100 ppm in order to prevent carbon monoxide poisoning to people.
On the other hand, when a gas sensor is used as a gas sensor for room environment analysis or a breath sensor for health management, the gas sensor needs to detect trace measurement target gas such as volatile organic compounds in the order of 0.001 to 10 ppm highly sensitively.
Japanese Patent Application Publication No. 2005-17242 (hereinafter referred to as “JP2005-017242”) and Japanese Patent Application Publication No. 2005-164566 (hereinafter referred to as “JP2005-164566”) disclose a gas leak detector for methane gas and a gas sensor for imperfect combustion. On the other hand, Japanese Patent Application Publication No. 2005-315874 (hereinafter referred to as “JP2005-315874”) discloses a gas sensor that is able to detect volatile organic compounds, combustible gas and poisonous gas.
According the invention of JP 2005-315874, a coating layer having ceramic particles of 8 to 30 nm is provided on an electrode and the coating layer is formed, for example of SnO2. According to JP2005-315874, the coating layer itself serves as a gas sensing layer that reacts with a measurement target gas and the measurement target gas is detected by change in resistance value of the coating layer. Thus, according to JP2005-315874, the gas detector has been improved in structure, but no improvement has been made to the layer structure for coating the surface of the gas sensing layer to increase the detection sensitivity.
In the structure where the gas sensing layer has been improved to improve the detection sensitivity as disclosed in JP2005-315874, there appears to be difficulty in improving the detection sensitivity for various kings of trace gases, depending on various purposes, simply and appropriately.
Conventionally there is no gas sensor that is capable of detecting trace measurement target gas such as volatile organic compounds highly sensitively.